I also think Interval Max is worth it (unless you can't get past the bit dated look: like the headband Cathe wears). It is my second-favorite Imax after IMAX2 (I didn't like the combo of way-high impact and more pivoty moves in IMax3, so I and II are the only ones I've kept...I'm hoping that an eventual Imax 4 is more like I and II!)
I think of it as very athletic, and more straightforward than IMax2 or Imax3 (which you wouldn't like if you like more choreography).
The music is fun (but if you prefer 'recognizable, top-40 tunes' you might not like it).